Wet, mild weather — not drought — may have helped Genghis Khan expand the Mongolian empire to the largest in human history.

Rings in trees in the mountains of central Mongolia show that at the same time the empire began to grow in the 13th century, the weather of central Asia became uncommonly wet. The moist climate could have boosted the region’s grasslands, giving Khan and his people more food for raising livestock and horses and a better ability to focus on building political and military power, researchers suggest March 10 in the Proceedings of the National Academy of Sciences.

Tree-ring and weather data also suggest that central Mongolia's drought from 2000 to 2009 was the hottest in the last 1,000 years. If warming continues, it could disrupt how modern Mongolians live, the scientists say.